Technical Note

Diffraction Contrast Tomography
Unlocking Crystallographic Informationfrom Laboratory X-ray Microscopy

Author: Carl Zeiss X-ray Microscopy, Inc.
Date:January 2019

X-ray tomography has operated under two primary contrast mechanisms for some time: X-ray absorption and phase
contrast, which both rely on material density differences within the sample. However, single-phase polycrystalline
materials (e.g. steels, alloys, etc.) do not exhibit any significant contrast using absorption or phase mechanisms.
Synchrotron-based XRM has demonstrated results in this area for about a decade with diffraction contrast tomography(DCT), which provides crystallographic/diffraction information from poly-crystalline samples, non-destructively, in 3D.
Now, advancing laboratory X-ray microscopy (XRM) one step further, we describe here the capabilities of laboratorybased DCT on the ZEISS Xradia 620 and 630 Versa 3D X-ray microscope, and the new research and characterization
capabilities this enables.

Introduction
In the continued spirit of transferring synchrotron capabilities

However, the destructive nature of 3D EBSD (via examinationto laboratory XRM systems, ZEISS in partnership with Xnovoof sequential slices) prevents one from directly evaluating theTechnology, has implemented the ability to obtain crystallo-

microstructure evolution when subject to either mechanical,

graphic (diffraction) contrast information on ZEISS Xradia 620

thermal or other environmental conditions. Understanding thisand 630 Versa[1,2].

evolution process on the same sample volume is key to unlocking
a more robust understanding of materials performance, along

Crystallographic imaging is commonly known from several metal-with improved modeling capabilities, and is a key driver of future

lographic techniques, including light and electron microscopy

materials research efforts.

(EM) methods. In recent years, the introduction of 2D and 3D
electron back-scattering diffraction (EBSD) techniques have madeIn response to this constraint, synchrotron-based crystallographic

EM a routine tool for research and/or development related toimaging, known as diffraction contrast tomography (DCT) has

metallurgy, functional ceramics, semiconductors, geology, etc.

emerged over the past decade [3,4]. Utilizing non-destructive

The ability to image the grain structure and quantify the crystallo-

X-rays, synchrotron users could quantify grain orientation infor-

graphic orientation relationships in such materials is instrumental

mation in the native 3D environment without physical sectioning.

for understanding and optimizing material properties (mechani-This has led to the logical desire to study the evolution of graincal, electrical, etc.) and in situ processing conditions.

crystallography in situ or during interrupted “4D” evolution
experiments. However, limited regular access to such synchrotron
techniques has constrained the ability to perform thorough,longitudinal studies on materials evolution.

Cover Image: LabDCT 3D imaging of grain orientations in aluminum alloy – IPF (inverse pole figure) colored.
Image Courtesy of: TU Denmark2

ZEISS has taken this established synchrotron modality andsuccessfully transferred it to the ZEISS Xradia Versa family oflaboratory-based sub-micron XRM systems. LabDCT may beefficiently coupled to in situ sample environments within themicroscope or subject to an extended time evolution “4D”
experiment (across days, weeks, months) – a unique practicalstrength of laboratory-based XRM/DCT experiments [5]. Following
an evolution experiment, the sample may be sent to the electron
microscope or focused ion beam (FIB-SEM) for post-mortem
complementary investigation of identified volumes of interest.
This natural correlative workflow combining multiple lab-based
methods on the same sample volume remains an attractive future
direction of microscopy to many researchers and is enabled by
the ZEISS foundation and breadth of imaging instrumentation.

Figure 1 Schematic of the LabDCT setup. The sample is illuminated through anaperture in front of the X-ray source. Both the sample absorption and diffraction
information is recorded with a high resolution detection system. Optionally, abeamstop maybe be added to the set-up in order to increase the dynamic range ofthe diffraction detection.

LabDCT: How it Works
Here, we present a method to acquire, reconstruct and analyze
grain orientation and related information from polycrystalline
samples on commercial laboratory X-ray microscopes (ZEISS
Xradia 620 and 630 Versa) that utilizes a synchrotron-styledetection system.
A schematic representation of the LabDCT implementationis shown in Figure 1. The X-ray beam is constrained througha specialized aperture to illuminate the sample. Diffraction information is collected with a tailor-made high resolution detector
that in addition allows simultaneous acquisition of the sample’sabsorption information. In order to increase the sensitivity to thediffraction signal a beamstop may be added. The data acquisitionis performed in a symmetric geometry, which enables improved
diffraction signal strength and allows handling of many andclosely located diffraction spots.
With LabDCT a series of projections is taken from which spatiallyresolved crystallographic information within the sample is derived. Figure 2 illustrates a single such 2D diffraction pattern as

Figure 2 Example 2D diffraction pattern from a Ti-alloy. The central area in theprojection is obscured by a beamstop. This missing data does not impact thediffraction analysis and reconstruction.

acquired with the LabDCT detector. Just a few hundred projections are enough for a complete LabDCT data set.

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Validation and Application Examples
Polycrystalline Ti-alloy (Timet 21S) and Al-alloy samples have beenba

explored and are used here to demonstrate the results of LabDCT.
Successful 3D reconstruction of the diffraction information
through Xnovo Technology GrainMapper3D™ software yields acompletely voxelated crystallographic representation of the entiresample. The information recovered from a successful reconstruction enables extraction of details of the sample’s microstructure,such as grains and their morphology, grain boundaries, grain

csizes, grain boundary normals, statistics across the entire sample,
pole figures for texture analysis, etc. A 3D grain map of a Tialloy sample is shown in Figure 3, where the grains are coloredaccording to their crystallographic orientation in relation to thevertical sample axis (inverse pole figure). A virtual slice throughthe sample is shown in Figure 3b, whereas a measure for the500 µm

quality of the LabDCT reconstruction is given to the user througha confidence map as seen in Figure 3c.

Figure 3 a) Visualization of a 3D grainmap of a titanium alloy (Timet 21S) fromreconstruction with the LabDCT GrainMapper3D software. Inverse pole figure color

To help validate the accuracy of the LabDCT technique, compari-coding highlights the crystallographic information. b) Virtual cross section throughsons to the closest related technologies, EBSD and synchrotron

the 3D grainmap; scale bar: 100 µm. c) Confidence map of the reconstruction

DCT and absorption X-ray microscopy were carried out. In doingso, independent measurements of sample microstructures werecorresponding to the virtual cross section in b); the gray scale reflects the confidence value of each voxel from 0% (black) to 100% (white) confidence.

obtained. Figure 4 shows the comparison of a 3D grain structurederived from LabDCT data (Figure 4a), with the 3D grain struc-acbdtures derived from high resolution X-ray absorption tomographyof an Al-alloy with segregated Cu on its grain boundaries.
Due to the density differences between Al and Cu the grainboundaries can be visualized through conventional X-ray
absorption tomography. Figure 4b displays the grain structureof the sample based on segmentation of the absorption contrast
dataset. The 3D grain structures agree remarkably well, withdifferences being located at the grain boundaries themselves
as shown through the difference map in Figure 4d. Main contributing factors to these differences are the segregation of theCu itself, leading to an uneven distribution and missing of somegrain boundaries (compare Figure 4c); furthermore, the subsequent segmentation of the grain boundary network from theabsorption tomography reconstruction will introduce uncertainties in the location of the boundaries.

250 µmFigure 4 a) LabDCT reconstruction of Al-Cu sample. b) Volumetric segmentationfrom absorption tomography. Colors in a) and b) indicate a grain index. c) Virtual
slice through the absorption tomography in gray-scale. d) Virtual slice of thedifference map between the data of a) and b); the binarized color scale indicatesregions of differences in white and regions with no difference in black.

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Also the angular accuracy of the crystallographic orientations

awere compared to those obtained using synchrotron DCT andEBSD. Some of the measurements are shown in Figure 5a alongwith the corresponding misorientations determined from the LabDCT measurement. The absolute values of the misorientation
obtained by the two techniques matches well within approximately one degree. Figure 5b shows a histogram of the differencein misorientation measured with EBSD versus LabDCT as well as
the difference in misorientations as measured by LabDCT versus
synchrotron DCT. The DCT techniques – both being X-ray based– differ in crystallographic orientation by less than 0.3 degrees,
whereas the differences between LabDCT and EBSD approach
one degree consistent with the lower angular resolution of theEBSD technique [6].
Conclusion
We have introduced the principles of diffraction contrast to-bmography and its application to determining crystalline grain
structure in samples. This illustrates the continued progress oflaboratory XRM to increase the diversity of imaging modalities
materials research and related fields. The unique ZEISS Xradia 620
Versa and 630 XRM hardware architecture enables data acquisition in powerful combination with advanced reconstruction andanalysis capabilities powered by Xnovo Technology and their

Occurrencesthat are inspired from synchrotron origins to solve problems inexperience in the field of DCT. The continued use and applications
development of this technique will accelerate the way 3D and 4D
science is pursued for non-destructively studying polycrystallinematerials.
Misorientation Difference [degree]
Figure 5 a) EBSD data from Ti-alloy sample with indications of misorientation angles
as determined by EBSD and LabDCT. b) Histogram of the differences in misorientation
as determined by LabDCT vs EBSD, and by LabDCT vs synchrotron DCT.

References:[1] S.A. McDonald, et al., Non-destructive mapping of grain orientations in 3D by laboratory X-ray microscopy, Scientific Reports, 5 		(2015) 14665.
[2] C. Holzner, et al., Diffraction Contrast Tomography in the Laboratory – Applications and Future Directions, Microscopy Today, July
2016, p. 34-42.
[3] H.F. Poulsen, Three-Dimensional X-ray Diffraction Microscopy (Springer, 2004)[4] W. Ludwig, P. Reischig, A. King, M. Herbig, E.M. Lauridsen, G. Johnson, T.J. Marrow and J-Y. Buffiere, Rev. Sci. Inst., 80, 033905, 		
(2009)[5] S.A. McDonald et al., Scientific Reports 7:5151 (2017)[6] D. Stojakovic, Processing and Applications of Ceramics, 6(1), 1-13, (2012)
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